How do I stop any future Tasks via a Group and limit the Task to only the people who are current members?

From the Manage Tasks view, click 3 pips next to the relevant Group Task and select ‘Delete Group task’.

You will see a pop-up appear with the option only to delete future and scheduled tasks.
If you select this option, Group task will no longer send out new tasks to members of this group, but it will also not delete old tasks, that were already issued.